Fuel cells have been proposed as a power source for many applications. One such fuel cell is the PEM (i.e., proton exchange membrane) fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a so-called "membrane-electrode-assembly" (hereafter MEA) comprising a thin (i.e., ca. 0.0015-0.007 inch), proton-conductive, polymeric, membrane-electrolyte having an anode electrode film (i.e., ca. 0.002 inch) formed on one face thereof, and a cathode electrode film (i.e., ca. 0.002 inch) formed on the opposite face thereof. Such membrane-electrolytes are well known in the art and are described in such as U.S. Pat. No. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia. In general, such membrane-electrolytes are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION.TM. available from the E.I. DuPont de Nemeours & Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material (e.g., NAFION.TM.) intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the present invention.
The MEA is sandwiched between sheets of porous, gas-permeable, conductive material which press against the anode and cathode faces of the MEA and serve as (1) the primary current collectors for the anode and cathode, and (2) mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh noble metal screen, and the like, as is well known in the art.
The thusly formed sandwich is pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors and conducting current between adjacent cells (i.e., in the case of bipolar plates) internally of the stack, and externally of the stack in the case of monopolar plates at the ends of the stack. The secondary current collecting plates each contain at least one so-called "flow field" that distributes the fuel cell's gaseous reactants (e.g., H.sub.2 and O.sub.2 /air) over the surfaces of the anode and cathode. The flow field includes a plurality of lands which engage the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply manifold at one end of the channel and an exhaust manifold at the other end of the channel. Serpentine flow channels are known and connect the supply and exhaust manifolds only after having made a number of hairpin turns and switch backs such that each leg of the serpentine flow channel borders at least one other leg of the same serpentine flow channel (see U.S. Pat. No. 5,108,849). Serpentine channels are advantageous in that they permit gas flow between adjacent legs of the same channel even when the flow channel is blocked (e.g., by a water droplet). In this regard, gas can flow from a portion of the channel which is upstream of the blockage where gas pressure is high, to a portion of the channel downstream of the blockage where gas pressure is low by flowing through the porous primary current collector over the land that separates a highly pressurized leg portion from a lowly pressurized leg portion of the next adjacent flow channel.
Prior art serpentine channels have heretofore promoted the loss of the gaseous reactants by locating the inlet leg of one serpentine flow channel adjacent the outlet leg of the next adjacent serpentine flow channel (see FIG. 4). Such an arrangement allowed reactants from the inlet leg (which is at relatively high pressure) of one flow channel to flow over the intervening land into the outlet leg (which is at relatively low pressure) of the next adjacent channel without traversing the entire length of the flow channel. In other words, some of the gas can short-circuit or bypass its intended flow channel and not be consumed in the fuel cell reaction.